The present invention relates generally to a method and an apparatus for controlling an electromagnetic clutch for use on a vehicle. More particularly, the invention is concerned with improved technologies for controlling an engaging action of such an electromagnetic clutch so as to permit a smooth start of the vehicle on a slippery road surface having a relatively low coefficient of friction.
In the art of a power transmission system of a vehicle, an electromagnetic clutch is used as means for easy control of a torque to be transmitted. For controlling an engaging action of the clutch, an electric current or voltage to be applied to energize a coil of the clutch is controlled according to a control program which is predetermined for maximum drivability and fuel economy of the vehicle at the time of its start. For example, the electric current or voltage to energize the coil of the electromagetic clutch is controlled in proportion to a difference between an idling speed of an engine of the vehicle and a driving speed of the engine. A coefficient of the proportion of the current or voltage to the difference in speed is constant, or determined as a function of an opening angle of a throttle valve of the engine, or other variables. An example of the above-indicated control is disclosed in Japanese Patent Application No. 58-131685 which was laid open on Feb. 6, 1985 under Publication No. 60-23639, which was assigned to the same assignee as the present application.
Such a known method of controlling the electromagnetic clutch as indicated above is practiced on the assumption that the road surface on which the vehicle is driven has a normal coefficient of friction. Accordingly, the known method suffers difficulty in starting the vehicle due to slippage of its drive wheels on the road surface, when the road surface has a very low coefficient of friction, e.g., when starting the vehicle on a frozen or sandy road surface or other slippery road surfaces.